Field of the Invention
The present invention relates to a liquid crystal display and a method for manufacturing the same.
Description of the Background Art
The common type of display method for use in liquid crystal displays has been the twisted nematic (TN) mode. Meanwhile, a transverse electric field type has been proposed in which voltage is applied between a pixel electrode and common counter electrodes (hereinafter also referred to as a “counter electrode” and a “common electrode”) to generate an approximately horizontal electric field in a panel and to drive liquid crystal molecules in a horizontal direction accordingly. The transverse electric filed mode has advantages in wide viewing angle, high definition, and high brightness, thus becoming mainstream for use especially in small-to-medium panels typified by smartphones and tablets. Known examples of the transverse electric field type include the in-plane switching (IPS)™ mode and the fringe field switching (FFS) mode. Specifically, a liquid crystal display employing the FFS mode includes a lower electrode and an upper electrode that is located on the lower electrode with an insulation film therebetween and has slits. One of the electrodes serves as a pixel electrode, whereas the other electrode serves as a counter electrode. Roughly speaking, an electric field for driving liquid crystals in such a configuration is generated in a manner to extend from the slits of the upper electrode toward liquid crystals above the upper electrode, to turn sideways in the liquid crystals, and then to extend toward the slits of the upper electrode.
A typical liquid crystal display includes a first substrate (a first insulation substrate) and a second substrate (a second insulation substrate) disposed to provide a certain amount of space therebetween and also includes a liquid crystal layer sandwiched between the first substrate and the second substrate. Provided in pixel areas on the first substrate side of the liquid crystal display employing the FFS mode are a plurality of signal lines, a plurality of scanning lines orthogonal to the signal lines, a plurality of thin film transistors located at intersections of the signal lines and the scanning lines, a pixel electrode, and a common electrode. Provided on the second substrate are a black matrix layer and a color filter layer. The black matrix layer blocks entry of light into the areas other than the pixel areas. The color filter layer is located on the portions of the second substrate corresponding to the individual pixel areas and consists of red (R), green (G), and blue (B) colors to produce the hue.
The first substrate of the liquid crystal display employing the FFS mode includes, in a display area of each pixel, a thin film transistor on a lower layer below the upper electrode and the lower electrode, with a protective insulation film located between the thin film transistor and these electrodes. Any given signal (voltage) from the outside is transmitted through the signal line to the thin film transistor, and then, is applied to the lower electrode or the upper electrode through a contact hole of the protective insulation film. The display area of each pixel corresponds to the area in which the upper electrode and the lower electrode overlap each other. A non-display area of each pixel corresponds to the area including the thin film transistor, the signal line, the scanning line, a common wire for reducing the resistance of the common electrode and the resistance distribution. The aperture ratio of the display area decreases with increasing proportion of the non-display area in each pixel, becoming difficulties in the production of high definition liquid crystal display. It is desired that the non-display area be minimized in order to achieve high definition.
Parasitic capacitance exists in this structure because the protective insulation film is sandwiched between the lower electrode and the signal lines. The parasitic capacitance may degrade the display quality. Thus, a liquid crystal display panel has been proposed in which an insulation film capable of reducing the parasitic capacitance is located on a lower layer below the lower electrode (Japanese Patent Application Laid-Open No. 2010-008758). The insulation film is an organic film having a small relative dielectric constant and a greater thickness. The organic film has excellent planarization properties, and thus, can smooth out irregularities caused by the thin film transistors. The organic film (hereinafter also referred to as an “organic planarization film”) may be photosensitive such that contact holes can be formed in the film in a photolithography process.
The liquid crystal display also includes spacers sandwiched between the first substrate and the second substrate and located in the non-display area. The spacers maintain a certain distance in the portion in which the liquid crystal layer is sealed. There are two types of spacers being a bead spacer and a columnar spacer. The bead spacers are spherical particles (beads) and distributed over the above-mentioned area, whereas the columnar spacers are fixed to the first substrate or the second substrate.
The beads spacers tends to move relatively freely after the first substrate and the second substrate are attached (bonded) to each other, so that some of the beads spacers may be mixed into the liquid crystals in the pixels. Consequently, the arrangement of the liquid crystal molecules around the spacers is disturbed, causing leakage of light in the relevant area. Unfortunately, this may reduce contrast and degrade the quality accordingly.
Unlike the beads spacers, the columnar spacers, which are fixed to the first substrate or the second substrate, do not reduce contrast. However, the liquid crystal display including sparsely arranged columnar spacers may fail to maintain the gap below the display surface touched by a hand or the like, causing irregularities. The restoration of the original state is time-consuming In the liquid crystal display including densely arranged columnar spacers, meanwhile, contraction of liquid crystals at a low temperature may not be accommodated by the distortion of the columnar spacers, so that a negative pressure may be applied to the inside of the liquid crystal panel. This may lead to low-temperature foaming. To eliminate the local distortion caused by an excessive load and the contraction of liquid crystals at a low temperature, a technique has been proposed (as disclosed in, for example, Japanese Patent No. 3925142) which uses two or more types of columnar spacers of different heights, such as main columnar spacers and sub columnar spacers, to keep the gap between the substrates.
The liquid crystals are aligned in alignment processing such as rubbing. This alignment processing is affected by steps caused by the columnar spacers, so that a leakage of light may occur and the alignment processing cannot be properly performed in some areas, namely, alignment abnormality areas. To eliminate or reduce the occurrence of these events, a technique has been proposed which uses columnar spacers placed in a light-shielding area (a non-display area).
In the above-mentioned configuration including the main columnar spacers and the sub columnar spacers of different heights as disclosed in Japanese Patent No. 3925142, some of these columnar spacers have larger diameters. In a high-definition liquid crystal panel, the alignment abnormality areas around the columnar spacers having larger diameters extend to a pixel display area, thereby causing degradation of quality, such as leakage of light and lower contrast.
Another technique has been proposed which uses a half-tone (HT) mask or a gray-tone (GT) mask as a photomask for forming columnar spacers. These masks enable control of the heights of columnar spacers based on the exposure value corresponding to the transmittance, so that columnar spacers having smaller diameters and different heights can be formed. Unfortunately, the production of the HT mask and the GT mask takes a longer period of time, thus causing a delay in starting the manufacturing of desired liquid crystal displays. The production of these masks involves complicated processes, which drive up costs of masks. This translates into an increase in the cost of liquid crystal displays (especially, the first substrate or the second substrate).